Overprint.

A fill or stroke flag that tells the RIP to print an object on top of what is beneath it. Grimco defines it in its Epson V-Series best practices as two colours overlapping and blending to make a new colour, against knockout, where the top colour cuts through the design underneath and stays true. Its example is a yellow rectangle on a magenta one: with Overprint Fill the two blend to red, without it the yellow cuts through. For white and varnish spot channels the flag matters more than for colour. Grimco says a White or Varnish spot not set to Overprint Fill or Overprint Stroke cuts through the design under it, and that design does not show whatever the Y-direction. The spot has to be named "White" or "Varnish", and its opacity sets ink density, so 70% opacity gives 70% density. See varnish for the clear channel. A white layer that is not set to overprint prints a hole where the colour should be.